Method And Apparatus For Abating Fugitive Emissions From A Volatile Liquid Storage Tank

ABSTRACT

A fugitive emissions abatement system for use a storage tank  4  with a floating roof assembly  10  comprises a roof structure  12,  a first peripheral seal  70  slidably engaging an interior surface  4 A of the tank  4  to form an annular gas chamber  60  intermediate the first peripheral seal  70  and the liquid level  50  within the tank  4,  and an aperture  78  in the first peripheral seal  70  coupled to the first end  82  of a fluid conduit  80.  The second end  81  of the fluid conduit  80  may be coupled to the inlet of a gas mover  88  so that operation of the gas mover  88  draws gas from the annular gas chamber  60  through the fluid conduit  80  so that it can be delivered to a processing apparatus that may include, but is not limited to, a compressor, a vessel, a heat exchanger and/or an incinerator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to an apparatus and a method to abate fugitive emissions from a volatile liquid storage tank. The present invention is directed to an improved floating roof assembly to abate fugitive emissions from a volatile liquid storage tank.

2. Description of the Related Art

Storage tanks are used to store volatile liquids including oil and other liquid hydrocarbons, produced brines, organic and non-organic chemicals, solvents, etc. Storage tanks may also be used to store sewage and other mixtures related to public utilities. In some storage tanks, the roof is not fixed, but instead it is adapted to be buoyantly supported by the liquid stored in the tank, and to rise and fall with the level of the liquid stored it the tank. The floating roof assembly may be buoyantly supportable within the interior of a liquids storage tank. For example, the floating roof assembly may be adapted with isolated pockets of trapped gas to make the floating roof assembly buoyant relative to the liquid in the tank, or it may comprise a roof structure that is made buoyant using pontoons or other buoyant members that may be secured to the roof structure and made a part of the floating roof assembly. A floating roof assembly may be buoyantly positionable by control of the volume of volatile liquid stored within the tank.

Volatile liquids may be introduced into and withdrawn from the storage tank to vary the elevation of the floating roof assembly within the tank. One or more pipes pipe may penetrate the wall of the storage tank, preferably near the bottom, or the one or more pipes may penetrate the floor of the storage tank, to introduce fluid into or to withdraw fluid from the storage tank.

A floating roof assembly may comprise a plurality of adjustable and deployable legs to prevent downward movement of the floating roof assembly beyond a certain height within the tank. For example, but not by way of limitation, the floating roof assembly may continue to descend with falling level of liquid in the tank until the feet located at the bottoms of the plurality of legs come to rest on the floor of the storage tank. In one embodiment, the plurality of legs may extend through the floating roof to prevent the roof from resting on or hitting the floor of the tank, or moving beyond a critical level beneath which the stored liquid is supplied to the tank or withdrawn from the tank. The plurality of support legs may be slidably adjustable in a substantially vertical orientation at right angles to the generally horizontal structure of the floating roof to provide a variable height at which the downward travel of the roof may be stopped. A seal may be used at each adjustable support leg penetration to prevent or abate fugitive gas emissions through or around each leg.

Floating roof assemblies may be used to generally isolate the contents of the liquids storage tank from the atmosphere. A conventional floating roof assembly may comprise one or more peripheral seals to obstruct evaporating gasses from escaping the tank and entering the atmosphere, but it is difficult to seal a conventional floating roof assembly against the interior of a tank that may exceed, for example, 100 meters in diameter. It should be appreciated by those skilled in the art that sealing an internal structure to prevent gas migration around such a large interface (the periphery of the floating roof assembly and the interior bore of the storage tank) is a challenge. The large size of the tank and the movable floating roof assembly makes machining close tolerances prohibitively expensive, and factors such as ambient wind direction and speed, ambient and stored liquid temperature, and uneven solar heating make conventional seals unusable on large floating roof-equipped tanks.

Some conventional floating roof assemblies have peripheral seals comprising gas-impermeable fabric skirts supported by framing on the roof structure. These peripheral seals generally slow and reduce, rather than substantially prevent, fugitive gas emissions.

What is needed is a method of abating fugitive emissions from a volume of stored volatile liquid within a large floating roof storage tank. What is needed is a floating roof assembly for a volatile liquid storage tank with an improved seal to abate fugitive emissions from the liquid stored in the tank. An apparatus and a method are needed to capture fugitive emissions prior to escape from a volatile liquids storage tank so that the fugitive emissions can be directed to a processing apparatus, such as a vessel, a condensation process or an incinerator.

SUMMARY OF THE INVENTION

The present invention satisfies one or more of the above-stated needs. This invention provides a fugitive emissions abatement apparatus and method. This invention provides a floating roof assembly to abate fugitive emissions from a volatile liquid storage tank. The present invention also provides, in one embodiment, a method for abating fugitive emissions from the periphery of a floating roof assembly installed within a volatile liquids storage tank by capturing and processing the fugitive emissions. The present invention may also provide, in another embodiment, an apparatus to capture fugitive emissions from the perimeter of a floating roof assembly installed within a liquids storage tank, and to direct the captured gas to a processing apparatus.

In one embodiment, a method comprises the steps of forming an annular gas chamber about a floating roof assembly to generally trap a volume of fugitive emissions within the annular gas chamber, and disposing a first end of a fluid conduit, such as a hose or a pipe, in fluid communication with the annular gas chamber. The method may further comprise the step of disposing a second end of the fluid conduit in fluid communication with an inlet to a gas mover, such as, but not limited to, a positive displacement compressor and/or a centrifugal compressor, such as, for example, a rotary fan. The method may further comprise the step of disposing an outlet of the gas mover in fluid communication with a fugitive gas processing apparatus including, but not limited to, a filter, an incinerator, a heat exchanger or a gas treating vessel, such as an absorption vessel containing a solid and/or liquid absorber.

In one embodiment, the floating roof assembly of the present invention may comprise a peripheral seals disposed about a roof structure to define an annular gas chamber, one or more apertures within the floating roof assembly in fluid communication with the annular gas chamber, and one or more fluid conduits, each having a first end and a second end, and each coupled at the first end to the one or more apertures and coupled at the second end to the inlet of one or more gas movers. In an optional embodiment, the apparatus may comprise a header having a plurality of inlets and at least one outlet, and the header may serve to aggregate a plurality of streams of fugitive emissions delivered to the header from a plurality of fluid conduits that may be coupled at their second ends to the plurality of inlets of the header. The header may be coupled at its outlet to a trunk line to the inlet of a gas mover, and the gas mover may have an outlet to deliver the aggregated fugitive emissions to a processing apparatus such as, but not limited to, a filter, an incinerator, a heat exchanger or a gas absorption vessel.

In an alternate embodiment, the floating roof assembly of the present invention may comprise a plurality of peripheral seals disposed about the roof structure to define a plurality of annular gas chambers, one or more apertures within the floating roof assembly, each aperture in fluid communication with at least one annular gas chamber, and one or more fluid conduits, each having a first end and a second end, and each coupled at the first end to one or more of the apertures and at the second end to an inlet of one or more gas movers. Optionally, one or more valves or other flow restrictors may be used to strategically distribute or allocate gas flow through a plurality of fluid conduits to impart a pressure differential between two or more annular gas chambers. In one embodiment, one or more of the valves may be coupled to an actuator for automatically positioning the valve and for automatically adjusting the rate of withdrawal from at least a portion of the annular gas chamber according to a sensed condition. In one embodiment, the sensed condition may be a differential pressure measured as the difference in pressure between the first annular gas chamber and one of the atmosphere or a second annular gas chamber adjacent to the first annular gas chamber. In an alternate embodiment, the sensed condition may be a concentration of a compound, such as oxygen, nitrogen, or a volatile compound emitted as a vapor from the volatile liquid being stored in the liquids storage tank.

So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. However, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section, elevation view of one embodiment of a floating roof assembly of the present invention having a first peripheral seal to slidably engage the interior wall of a volatile liquid storage tank and to define an annular gas chamber there below, an aperture within the floating roof assembly in fluid communication with the annular gas chamber, and a fluid conduit coupled at its first end to the aperture.

FIG. 2 is a cross-section, elevation view of an alternate embodiment of a floating roof assembly of the present invention having a first peripheral seal to slidably engage the interior wall of a volatile liquid storage tank and to define a first annular gas chamber there below, an aperture within the floating roof assembly in fluid communication with the first annular gas chamber, a second peripheral seal secured to slidably engage the interior wall of the tank and to define a second annular gas chamber there below, and a fluid conduit coupled at its first end to the aperture through the first peripheral seal.

FIG. 3 is a cross-sectional prospective view of the embodiment of a floating roof assembly of FIG. 1 revealing a seal flange coupling the first peripheral seal to the roof structure of the floating roof assembly, and a lip flange coupling a peripheral lip to the first peripheral seal.

FIG. 4 is a top plan view of an embodiment of a floating roof assembly of the present invention comprising a plurality of fluid conduits, each having a first end and a second end, the fluid conduits each coupled at the first end to an aperture in the first peripheral seal to dispose the fluid conduit in fluid communication with an annular gas chamber disposed about the periphery of the floating roof assembly and defined by a peripheral seal. The second end of each fluid conduit is coupled to a gas header.

FIG. 5 is an enlarged view of the header in FIG. 4, the header having a plurality of inlets, each to receive a stream of fugitive emissions from the plurality of fluid conduits, and a header discharge to deliver the aggregated fugitive emissions received into the header from the fluid conduits to a gas processing apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In one embodiment of the apparatus of the present invention comprises a floating roof assembly comprising a generally disk-shaped buoyant roof structure having a first peripheral seal disposed about its periphery to engage the interior wall of a volatile liquid storage tank and to define an annular gas chamber about the roof structure and generally below the first peripheral seal. The embodiment further comprises at least one aperture penetrating a component of the floating roof tank assembly that is coupled to a first end of at least one fluid conduit. A second end of the at least one fluid conduit is coupled to the inlet to a gas mover, such as a positive displacement compressor or a centrifugal compressor, such as a fan, to continuously withdraw gas from the annular gas chamber.

As fugitive gas is continuously withdrawn from the annular gas chamber, a small vacuum, preferably less than about 0.15 inches of water, is imparted to the annular gas chamber as a result of the continuous withdrawal of gas. The vacuum resulting from the continuing withdrawal of gas from the annular gas chamber may cause atmospheric air to enter the annular gas chamber at locations along the first peripheral seal that may be separated from or inadequately engaging the interior wall of the liquids storage tank. As a result, the gas withdrawn from the annular gas chamber, and routed to the inlet of the gas mover, may be a mixture of air and volatile gas, and may be explosive. Preferably, the gas mover used to draw the vacuum on the annular gas chamber and to route the withdrawn gas to a processing apparatus is of a non-sparking design with an explosion proof driver, and is designed to isolate or prevent sparks or other sources of ignition from the explosive mixture.

In a preferred embodiment, a plurality of apertures penetrate the floating roof assembly, and each aperture is coupled to the first end of a fluid conduit, such as a hose or pipe, that is coupled at its second end to the inlet of the gas mover. As a result, gas may be withdrawn from a plurality of apertures that may be angularly distributed along the periphery of the floating roof assembly. This preferred embodiment may provide a more uniform distribution of the vacuum in the annular gas chamber, and an overall improved abatement of the fugitive emissions escaping the floating roof assembly and entering the atmosphere. It should be understood, however, that localized seal deficiencies may make it more advantageous to unevenly distribute the vacuum to withdraw more gas from portions of the annular gas chamber adjacent to portions of the peripheral seal where the peripheral seal performs poorly, and to withdraw less gas from portions of the annular gas chamber adjacent to portions of the peripheral seal where the peripheral seal performs well. This uneven distribution may be implemented using adjustable valves disposed in fluid communication with one or more of the fluid conduits in embodiments of the present invention comprising a plurality of fluid conduits. It should be understood that, using a gas mover with an inlet coupled to a plurality of fluid conduits, closing a given adjustable valve to constrict and decrease flow through one fluid conduit may cause an increased rate of flow through one or more of the remaining gas conduits that are also in fluid communication with the inlet to the gas mover.

The present invention also provides a method of monitoring and assessing performance of one or more portions of the peripheral seal disposed about the periphery of the floating roof assembly. In one embodiment, a sample of gas may be taken from a first fluid conduit withdrawing gas from a first portion of the annular gas chamber disposed about the periphery of the floating roof assembly, and a second sample of gas may be taken from a second fluid conduit withdrawing gas from a second portion of the annular gas chamber, and the two gas samples may be analyzed, for example, but not by way of limitation, using chromatographic analysis. The chromatographic analysis may reveal, for example, that the first gas sample representative of the composition of gas within the first portion of the annular gas chamber contains a substantially greater oxygen and/or nitrogen content than the second gas sample representative of the composition of gas within the second portion of the annular gas chamber. These results would indicate that the portion of the first peripheral seal adjacent to the first portion of the annular gas chamber performs poorly compared to the portion of the first peripheral seal adjacent to the second portion of the annular gas chamber. Remedial repairs, modifications or further investigations or tests may be made or performed as a result of these findings in order to repair, modify or adjust the first peripheral seal and to thereby abate the release of fugitive emissions into the atmosphere.

In an alternative embodiment, an apparatus may comprise a floating roof assembly comprising a generally disk-shaped buoyant roof structure having a first peripheral seal disposed about its periphery to engage the interior wall of a volatile liquid storage tank and a second peripheral seal disposed about its periphery to engage the interior wall of the volatile liquid storage tank in a spaced-apart relationship from the first peripheral seal to define a first annular gas chamber there between. This dual-seal arrangement may also define a second annular gas chamber between the second peripheral seal and the surface of the liquid. The alternative embodiment may further comprise one or more apertures penetrating the floating roof assembly to facilitate the withdrawal of gas from the first annular gas chamber. In yet another alternative, the dual-seal arrangement floating roof assembly may comprise one or more apertures penetrating the floating roof assembly to facilitate withdrawal of gas from the second annular gas chamber. In yet another alternative embodiment, there may be two or more apertures penetrating the floating roof assembly, one or more to facilitate the withdrawal of gas from the first annular gas chamber, and one or more to facilitate the withdrawal of gas from the second annular gas chamber.

An embodiment of the apparatus of the present invention may comprise a gas mover having an inlet and an outlet. The gas mover may include, for example, one or more positive displacement compressors and/or one or more centrifugal compressors, such as a rotary fan. A plurality of gas movers may be connected in series or in parallel, depending on the volume and the head needed to move the fugitive emissions to the processing apparatus. The inlet(s) to the gas mover(s) may be fluidically coupled through at least one fluid conduit to the at least one aperture that provides fluid communication with an annular gas chamber. The inlet, when placed in communication with the at least one annular gas chamber through a fluid conduit, will draw gas from one or more annular gas chambers. It should be understood that two or more gas movers may be used to obtain the targeted withdrawal rate(s) and/or vacuum(s) within the one or more annular gas chambers. It should further be understood that the use of one or more gas movers may enable the operator to maintain a desired imbalance or balance between any two of the atmosphere and the one or more annular gas chambers. For example, but not be way of limitation, an operator may wish to abate fugitive emissions and also to ensure that air does not contaminate the liquid product. An operator may utilize a dual-seal arrangement as described above for the floating roof assembly to form two generally adjacent annular gas chambers, and to further ensure that a generally continuous pressure differential from the second annular gas chamber defined between the second seal and the liquid surface to the first annular gas chamber between the first seal and the second seal. The operator may use valves, for example, to modulate or limit the rate of gas drawn from the second annular gas chamber while drawing more gas from the first annular gas chamber, and thereby provide a net pressure differential from the second annular gas chamber to the first annular gas chamber to prevent gas from the first annular gas chamber, which may contain more air, from flowing to the second annular gas chamber where it can contact the surface of the liquid stored in the tank. It should be understood that a reversed pressure differential may be established and maintained in the direction from the first annular gas chamber to the second annular gas chamber where, for example, an operator desires to prevent the release of fugitive emissions to the atmosphere, as a reversed pressure differential from the first annular gas chamber to the second annular gas chamber will better ensure that fugitive emissions from the liquid stored in the tank are better drawn into the fugitive emissions abatement system of the present invention rather than entering the first annular gas chamber which may be separated from the atmosphere only by the first peripheral seal.

It should be understood that the structures that comprise the first peripheral seal and the second peripheral seal may vary. It is preferred that the structures that comprise these gas barriers are generally flexible, as opposed to being rigid and unbendable, so that the first peripheral seal and/or the second peripheral seal may conform to the shape of the interior wall of the liquid storage tank. It may also be preferred, in some applications, that the structures comprise materials that will not generate sparks or otherwise ignite a combustible gas mixture. For example, but not by way of limitation, portions of the first peripheral seal and/or the second peripheral seal that may rub against a metal interior wall of the tank may comprise a non-sparking material, such as fiberglass, rubber, plastic, or bronze and some aluminum alloys. Alternately, a non-sparking coating may be used to suppress sparks.

In one embodiment, the first peripheral seal and/or the second peripheral seal may comprise a skirt portion and a lip portion, and the skirt portion may comprise a gas impermeable barrier such as sheet metal, fiberglass, plastic or other material. In an alternate embodiment, the skirt may comprise a permeable and flexible material, such as expanded metal, with a gas impermeable liner, such as rubber, vinyl or plastic. A lip may be disposed along the portion of the first peripheral seal and/or the second peripheral seal that contacts the interior wall of the tank, and the lip may be of a flexible and resilient material, such as rubber. In one embodiment, the lip is releasably securable to the skirt.

FIG. 1 is a cross-sectional, elevation view of one embodiment of a conventional volatile liquid storage tank 4 having an interior wall 4A. A floating roof assembly 10 comprising a generally disc-shaped roof structure 12 (only a portion of the edge of the roof structure 12 is shown in FIG. 1) is received within the bore of the tank 3. The floating roof structure 12 comprises isolated gas pockets 13 for imparting buoyancy to the roof structure 12. The floating roof assembly 10 of FIG. 1 further comprises a generally sloped rainwater collection surface 14 to gather and converge rainwater toward a drain 15 (not shown in FIG. 1—see FIG. 4) near the center of the roof structure 12. The drain may be fluidically coupled to a flexible drain pipe disposed underneath the roof structure 12 that is adapted to isolate collected rainwater from the contents of the liquid storage tank 4, and to deliver collected rainwater from the tank notwithstanding movement of the floating roof assembly 10 within the liquid storage tank 4.

The floating roof assembly 10 may further comprise one or more stabilizers 40 to maintain a desired minimal stand-off between the roof structure 12 and the tank 4. The stabilizer 40 may comprise a roller 42 that is pivotal on a pin 44 to provide smooth relative movement between the roof structure 12 and the interior wall 4A of the tank 4. In one embodiment, the roller 42 may be extendable from the stabilizer 40 to provide adjustable positioning of the roof structure 12.

The floating roof assembly 10 of FIG. 1 further comprises a first peripheral seal 70 secured to the floating roof assembly 10 to seal the periphery of the disc-shaped structure 12 against the interior wall 4A of the tank 3. In the embodiments shown in FIGS. 1-4, the first peripheral seal 70 is adapted for slidably engaging the interior surface 4A of the wall 4 of the tank. The first peripheral seal 70 may comprise a plurality of seal sections 74A (not shown in FIG. 1—see FIG. 4) coupled together to form a skirt 74 that may extend about the periphery of the roof structure 12. The skirt 74 of the first peripheral seal 70 may comprise a thin and flexible material, such as sheet metal, expanded metal, plastic or vinyl. In the embodiment shown in FIG. 1, the first peripheral seal 70 comprises a dual-layered skirt 74 that includes a skirt liner 75 that may comprise a gas impermeable material such as rubber, vinyl, plastic or sheet metal. It should be understood that a peripheral seal that comprises a skirt 74 may comprise a single layer skirt or a skirt having multiple layers. It should further be understood that although a first peripheral seal having a skirt is used to illustrate an embodiment of the invention, the invention may comprise and may be implemented using a peripheral seal that does not comprise a skirt, and the peripheral seals shown in the appended drawings are used merely to illustrate the invention, and should not be considered limiting of the invention. Other peripheral seals that may be used to form or implement embodiments of the invention may comprise, but do not necessarily comprise, inflatable bladders as sealing elements, hydraulically biased sealing elements, and/or spring-biased sealing elements. Additionally, sealing elements may comprise compressible materials such as elastomers or other generally flexible and resilient materials.

The floating roof assembly 10 shown in FIG. 1 is generally positionable within the tank 4 by control of the liquid level 50 of the liquid stored within the tank 4. It should be understood that the roof structure 12 is buoyant, and that the location of the liquid level 50 on the floating roof assembly 10 will be determined by the size, weight and effective density of the floating roof assembly 10, its areal extent at the liquid level 50, the density of the liquid 6 stored in the tank 4, and the weight of apparatuses and articles placed on and supported by the floating roof assembly 10.

As shown in FIG. 1, a first annular gas chamber 60 may be formed between the first peripheral seal 70 and the liquid level 50. One or more apertures 78 through the skirt 74 of the first peripheral seal 70 are each positioned to provide fluid communication between the first end 82 of a fluid conduit 80 and the first annular gas chamber 60. The floating roof assembly 10 shown in FIG. 1 comprise only one fluid conduit 80 and only one aperture 78, but it should be understood that a plurality of fluid conduits 80 may each be fluidically coupled to an aperture 78 in the first peripheral seal 70. The aperture 78 in FIG. 1 is coupled to the first end 82 of the fluid conduit 80 to provide fluid communication between an inlet to a gas mover (not shown in FIG. 1—see FIG. 4) and the first annular gas chamber 60 formed underneath the first peripheral seal 70 and generally about the roof structure 12. It should be appreciated by those skilled in the art that connecting the second end 81 of the fluid conduit 80 (second end 81 not shown in FIG. 1—see FIG. 4) to the inlet of a gas mover will draw gas from the annular gas chamber 60 and into the gas mover.

As described above, the floating roof assembly 10 of the present invention may be disposed in a very large tank, and the first peripheral seal 70 may comprise numerous seal sections 74 (see FIG. 4) coupled to form a single peripheral seal. One method of the invention may be implemented by sampling the gas in one or more of the fluid conduits 80 that may be used to draw gas from one or more annular gas chambers. One or more gas sampling ports 80A (see FIG. 1) may be inserted within one or more fluid conduits 80 of the floating roof assembly 10 of the present invention. Each gas sampling port 80A may comprise a “T”-shaped fluid conduit or fitting with a straight-through portion aligned with the fluid conduit 80 and an intersecting fluid leg terminating in a valve so that, by fluidically coupling a sample bomb or a sample cylinder to the valve of the gas sampling port 80A and then opening the valve, a representative sample of the gas being withdrawn from the first annular gas chamber 60 through an aperture 78 and drawn to the gas mover (not shown in FIG. 1—see FIG. 4) through the fluid conduit 80 may be obtained for analysis.

Factors such as material seal degradation, ambient wind or temperature, solar heating or wear may cause certain seal sections 74A to fail or to underperform relative to adjacent or other seal sections 74A. In one embodiment of the method, comparison of gas samples obtained using the sampling ports 80A may indicate poorly performing portions or sections of the first peripheral seal or other peripheral seals to be discussed below in connection with FIG. 2. For example, but not by way of limitation, the gas obtained using the sampling port 80A in a first fluid conduit 80 used to draw gas from a first section of the annular gas chamber 60 may be analyzed and determined to contain a substantially greater concentration of a compound or element, such as, for example, oxygen, than a sample of gas obtained using another sampling port 80A in a second fluid conduit 80 used to draw gas from a second section of the annular gas chamber 60. The substantially greater concentration of the compound or element may indicate a localized seal failure or other condition requiring repair, replacement or further investigation.

FIG. 2 is a cross-sectional, side elevation view of an alternate embodiment of a floating roof assembly 10 having a first peripheral seal 70 secured to the roof structure 12 to slidably engage the interior wall 4A of the tank 4 at a first contact 52, a second peripheral seal 71 secured to the roof structure 12 to also slidably engage the interior wall 4A of the tank 4 at a second contact 52B in a spaced-apart relationship to the first contact 52A. FIG. 2 also shows a stabilizer 40 that may be spaced from the roof structure 12 by stabilizer legs 49 to prevent interference between the roller 42 and the second peripheral seal 71.

The first peripheral seal 70 and the second peripheral seal 71 define a first annular gas chamber 60 there between. Like the first peripheral seal 70, the second peripheral seal 71 may be coupled to or disposed on the roof structure 12 to seal the periphery of the roof structure 12 against the interior wall 4A of the tank 4. In the embodiment shown in FIG. 2, the first peripheral seal 70 and the second peripheral seal 71 are both adapted to slidably engage the interior surface 4A of the wall of the tank 4. The first peripheral seal 70 and the second peripheral seal 71 may both be generally circular in shape and of a generally equal areal extent along circular contacts 52A, 52B, and the second peripheral seal may also comprise a plurality of coupled seal sections, similar to the seal sections 74A (not shown in FIG. 1—see FIG. 4) of the first peripheral seal 70, coupled one to the others, and the first peripheral seal 70 and the second peripheral seal 71 may both extend around the periphery of the floating roof assembly 10 in a spaced apart relationship to define a first annular gas chamber 60 there between.

The second peripheral seal 71 may, like the first peripheral seal 70, comprise a skirt that may be formed of a thin and flexible material, such as sheet metal, plastic or vinyl. In the embodiment of the second peripheral seal 71 shown in FIG. 2, the second peripheral seal 71 comprises a single-layer skirt comprising a gas impermeable material such as rubber, vinyl, plastic or sheet metal. It should be understood that the floating roof assembly 10 of FIG. 2 may comprise any arrangement of seal elements including one or more single layered skirts, like the second peripheral seal 71 depicted in FIG. 2, one or more dual-layered skirts, like the first peripheral seal 70 depicted in FIGS. 1-3, inflatable bladders, spring-biased sealing elements, or any other seal elements or combinations of seal elements that may be used to seal the roof structure 12 with the interior wall 4A of the tank 4. It should be further understood that the structure of the peripheral seals 70, 71 disclosed in the embodiments of the invention disclosed herein should not be considered as limiting of the scope of the invention, as any peripheral seal that may be used to define one or more annular gas chambers about the periphery of a floating roof structure may be used to make the apparatus of the invention or to implement the method of the invention.

Returning to FIG. 2, the second peripheral seal 71 may be, like the first peripheral seal 70, coupled to the roof structure 12 to slidably engage the interior wall 4A of the tank 4 at seal contact 52B, and to define a second annular gas chamber 62 between the second peripheral seal 71 and the liquid level 50 and about the roof structure 12. The floating roof assembly 10 of FIG. 2 further comprises a first fluid conduit 80 having a first end 82 fluidically coupled to a first aperture 78 through the floating roof assembly 10. As shown in the illustration of FIG. 2, the aperture may be disposed through the first peripheral seal 70 to place the inlet of a gas mover (not shown in FIG. 2—see FIG. 4) in fluid communication with the first annular gas chamber 60. Optionally, as shown in FIG. 2, the floating roof assembly 10 of FIG. 2 may further comprise a second fluid conduit 90 having a first end 92 fluidically coupled to a second aperture 79A and, through an auxiliary fluid conduit 94, to a third aperture 79B, to place the inlet of a gas mover (not shown in FIG. 2—see FIG. 4) in fluid communication with the second annular gas chamber 62. In FIG. 2, the aperture 79A is through the rainwater collection surface 14 and the aperture 79B is through a side wall of the roof structure 12, but it should be understood that there are numerous routes and variations of methods of placing the inlet of the gas mover in fluid communication with the second annular gas chamber including, but not limited to, extending a fluid conduit through the first peripheral seal 70 and then through the second peripheral seal 71. It should be understood that, using the locations of the apertures 79A, 79B as shown in FIG. 2 or using alternative locations, the inlet to a gas mover can be fluidically coupled to either one or both of the annular gas chambers 60, 62 defined by the first and second peripheral seals 70, 71 shown in FIG. 2. It should further be understood that placing the first fluid conduit 80 in fluid communication with the inlet to a gas mover and the second fluid conduit 90 in fluid communication with the same inlet to a gas mover or with an inlet to a second gas mover will result in gas being drawn from the first annular gas chamber 60 and from the second annular gas chamber 62, and gas will flow in fluid conduit 80 in the direction of arrow 81′, and gas will flow in the second fluid conduit 90 in the direction of arrow 91′.

FIG. 3 is a cross-sectional perspective view of the embodiment of the floating roof assembly 10 shown in FIG. 1 showing a portion of the first peripheral seal 70 coupled to the roof structure 12 along a skirt flange 73. The roof structure 12 comprises a skirt receiving flange 17 disposed generally along its periphery and positioned to receive and couple to the skirt flange 73 of the first peripheral seal 70 using conventional fasteners, such as bolts 77. The first peripheral seal 70 may further comprise a peripheral lip 79 having a lip flange 76 coupled to a lip receiving flange 18 on the skirt. The lip 79 may comprise a resilient rubber material that, when urged against the interior wall 4A of the tank 4, may generally conforms to the shape of the interior wall 4A to provide a seal to abate leakage of fugitive emissions from the tank to the atmosphere.

FIG. 4 is a top plan view of one embodiment of a floating roof assembly 10 comprising a plurality of fluid conduits 80, each having a first end 82 that is located at or near the first peripheral seal 70 and in fluid communication with one or more annular gas chambers (not shown in FIG. 4—see elements 60 and 62 in FIGS. 1-3), and a second end 81 in fluid communication with a header 85. The second end 81 of each fluid conduit is shown fluidically coupled to a header 85 comprising a plurality of inlets 85A and a discharge 85B that is fluidically coupled to an inlet 87A of a header/gas mover conduit 87. The header/gas mover conduit 87 further comprises an outlet 87B fluidically coupled to the inlet 88A to a gas mover 88. The gas mover 88 further comprises an outlet 88B coupled to a discharge trunk line 89 that carries the discharged fugitive emissions to a processing apparatus that may include, but is not limited to, a compressor, a vessel, a heat exchanger and/or an incinerator. FIG. 5 is an enlarged view of the header 85 shown in FIG. 4, the header 85 having a plurality of inlets 83 to receive fugitive emissions from the plurality of second ends 81 of the plurality of fluid conduits 80, and a header outlet 85B coupled to the inlet 89A to the header/gas mover conduit 89.

FIG. 5 is an enlarged view of a header that could be used, for example, in FIG. 4, to aggregate the streams of gas withdrawn through a plurality of fluid conduits for discharge to a single gas mover or to a series or bank of gas movers, as is necessary to achieve the desired flow and withdrawal rates. The header 85 may comprise a plurality of inlets 81, each for discharging the gas stream from a corresponding fluid conduit 80, and each inlet 81 coupled to a valve 83 that is in fluid communication between the inlet 81 and the header/gas mover conduit 87. Each valve 83 may have an open position for flow therethrough, and a closed position to terminate flow. Alternately, each valve 83 may have an open position for flow therethrough, and a partially open position to restrict flow therethrough. It should be understood that, by the nature of fluid mechanics, that a flow restriction applied by the closure or restriction of one valve 83 will, assuming steady operation of the gas mover, result in adjustments in the fluid conduit 80 coupled to that adjusted valve 83, and also in all other fluid conduits that access the same gas mover inlet through a header system that includes that adjusted valve.

It should be understood that the valves 83 may be actuated valves that are positionable using a valve actuator. In one embodiment, one or more of the valves may be actuated using an actuator that positions the valve in response to a signal generated by an instrument, such as a differential pressure transmitter, a pressure sensor, a gas chromatograph or a temperature sensor. In one embodiment, a valve actuator comprising, but not limited to, an electrically, pneumatically, or hydraulically operated valve, may be used to automatically adjust the rate of gas withdrawal from one or more portions of a first annular gas chamber or a second annular gas chamber, or both. In one embodiment, one or more of the valves may be coupled to an actuator for automatically positioning the valve and for automatically adjusting the rate of withdrawal from at least a portion of the annular gas chamber according to a sensed condition. In one embodiment, the sensed condition may be a differential pressure measured as the difference in pressure between the first annular gas chamber and the atmosphere. In an alternate embodiment, the sensed condition may be a differential pressure measured as the difference in pressure between the first annular gas chamber and a second annular gas chamber adjacent to the first annular gas chamber. In another alternate embodiment, the sensed condition may be a concentration of a compound, such as oxygen, nitrogen, or a volatile compound emitted as a vapor from the volatile liquid being stored in the liquids storage tank. It should be understood that, while differential pressure transmitters, gas chromatographs, pressure sensors and temperatures sensors may be used, standing alone or in combination, to generate signals to automatically control the gas withdrawal rates using automatically positionable actuated valves, the valves may be manually adjusted based on the sensed conditions using these same instruments, and a lack of automation is still within the scope of the claims.

It should be understood that the pressurized and aggregated fugitive emissions discharged from the outlet of the gas mover may flow to an apparatus to recover, incinerate or process the fugitive emissions in a manner that is more substantially less environmentally damaging compared to the release of unprocessed fugitive emissions from the tank. It should be further understood that a variety of structures may be used to create one or more annular gas chambers, and to fluidically couple the annular gas chamber(s) to a gas mover. It should be understood that the annular gas chambers may be divided or segregated, or combined or aggregated to form smaller or a larger chamber without departure from the spirit and purpose of the invention, and without diminishing the effectiveness of the invention.

The terms “comprising,” “including,” and “having,” as used in the claims and specification herein, shall be considered as indicating an open group that may include other elements not specified. The terms “a,” “an,” and the singular forms of words shall be taken to include the plural form of the same words, such that the terms mean that one or more of something is provided. The term “one” or “single” may be used to indicate that one and only one of something is intended. Similarly, other specific integer values, such as “two,” may be used when a specific number of things is intended. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. 

1. A method of abating fugitive emissions from a volatile liquids storage tank comprising the steps of: providing a generally disc-shaped floating roof assembly having an annular recess within the volatile liquids storage tank; disposing a seal about the floating roof assembly in a vertically spaced-apart relationship from a liquid interface on the floating roof assembly to define an annular gas chamber there between; and disposing the annular gas chamber in fluid communication with an inlet to one or more gas movers through one or more fluid conduits to withdraw gas from the annular gas chamber.
 2. The method of abating fugitive emissions from a volatile liquids storage tank of claim 1 further comprising the step of: disposing an outlet of the one or more gas movers in fluid communication with an inlet to a volatile liquids recovery process.
 3. The method of abating fugitive emissions from a volatile liquids storage tank of claim 1 further comprising the step of: disposing an outlet of the one or more gas movers in fluid communication with an inlet to a volatile liquids incineration process.
 4. The method of controlling fugitive emissions from a volatile liquids storage tank of claim 1 further comprising the step of: biasing the seal against an interior wall of the volatile liquids storage tank.
 5. The method of claim 1 further comprising the step of coupling the gas mover to the floating roof assembly.
 6. A method of controlling fugitive emissions from a volatile liquids storage tank comprising the steps of: providing a generally disc-shaped floating roof assembly having an annular recess within the volatile liquids storage tank; disposing a first generally circular seal about the floating roof assembly in a vertically spaced-apart relationship from a liquid interface about the floating roof assembly to define a first annular gas chamber there between; disposing a second generally circular seal about the floating roof assembly in a vertically spaced-apart relationship from the first generally circular seal to define a second annular gas chamber there between; and disposing at least one of the first annular gas chamber and the second annular gas chamber in fluid communication with an inlet to one or more gas movers through one or more fluid conduits.
 7. The method of controlling fugitive emissions from a volatile liquids storage tank of claim 6 further comprising the step of: disposing an outlet of the one or more gas movers in fluid communication with an inlet to a volatile liquids recovery process.
 8. The method of controlling fugitive emissions from a volatile liquids storage tank of claim 4 further comprising the step of: biasing the generally circular seal against an interior wall of the volatile liquids storage tank.
 9. The method of controlling fugitive emissions from a volatile liquids storage tank of claim 1 further comprising the step of: periodically sampling the gas drawn from the annular gas chamber through the one or more fluid conduits to determine the performance of the first seal.
 10. The method of controlling fugitive emissions from a volatile liquids storage tank of claim 9 further comprising the step of: containing at least a portion of gas emitted from a volume of volatile liquid stored in the storage tank using a seal disposed generally around a disc-shaped floating roof assembly; withdrawing at least one sample of gas from a plurality of locations along an annular gas chamber disposed about the floating roof assembly; and comparing gas samples drawn from a plurality of angularly distributed fluid conduits to assess the performance of identifiable portions of a seal.
 11. A method of controlling fugitive emissions from a volatile liquids storage tank comprising the steps of: withdrawing one or more streams of gas from an annular gas chamber around a floating tank roof assembly using a gas mover to maintain a vacuum within the annular gas chamber; and processing the one or more gas streams to minimize emissions to the atmosphere from the tank.
 12. The method of claim 11 further comprising sampling the one or more gas streams to determine the performance of identifiable portions of the seal.
 13. A floating tank roof assembly comprising: an annular recess about a first perimeter portion of a roof platform; a first seal about a second perimeter portion to form a first annular chamber there below; and one or more fluid conduits having an inlet at the first annular gas chamber an outlet in and the suction of a gas mover to withdraw gas from the annular chamber.
 14. The floating roof tank assembly of claim 13 further comprising one or more sampling ports accessing one or more fluid conduits.
 15. The floating roof assembly of claim 13 further comprising a second seal about a third perimeter portion of the roof platform.
 16. The floating roof assembly of claim 15 wherein the second seal forms a second seal forms a second chamber above the first annular chamber.
 17. The floating assembly of claim 13 further comprising one or more tracer gas injection ports.
 18. The floating tank roof assembly of claim 13 further comprising one or more automatic gas sampling devices.
 19. The floating tank roof assembly of claim 18 further comprising a means for transmitting one or more signals indicating the content of gas withdrawn from the first annular gas chamber.
 20. The method of claim 11 further comprising the steps of: providing at least one valve in fluid communication between the annular gas chamber and the gas mover; sensing a condition of the annular gas chamber and generating a signal; adjusting the at least one valve in response to the signal to modulate the rate of gas withdrawal from the annular gas chamber through the one or more gas streams. 